The implication in the question is that "Dave" needs to be focused in order to visualize the target and access it through teleportation.
So the first thing to do is break his ability to focus and concentrate. Perhaps the most effective means without using permanent sedation or lobotomizing the character (and why isn't this allowed?) would be to place him in the "white room".
The British Army pioneered this technique during "the Troubles" against the PIRA in the 1970's in response to charges that they were using torture against PIRA terrorists. The room is simply a very small enclosure which is painted a stark white colour, has 24hr fluorescent lighting and is constantly filled with "white noise" to obscure any aural cues coming from outside.
The techniques was refined by ensuring no time cues are ever provided either; meals arrive at random times, the same person always deliver meals or changes of clothes and bedding, the same interrogators come to visit...you get the idea.
With nothing to "attach" to, the mind begins to wander and become unfocused. Since the meals and interrogations occur at random intervals, normal sleep cycles are interrupted as well, further disrupting mental processes. Variations of this include PSYOPS units using powerful loudspeakers to blast atonal sounds at enemy positions to disrupt sleep and impair communications (trying to give orders while screaming at the top of you lungs isn't very effective for communicating complex ideas or plans).
A fallback position would be to identify every place he has ever been and attempt to demolish or drastically rearrange the interior of the structure so his memory of the place is faulty. Arranging matters so he will teleport into the interior of a wall will either be instantly fatal or discourage further escape attempts, especially if he is informed in no uncertain terms that this has been done (but not specifying which places have been changed and which might still remain as they were). The combination of lack of focus and doubt that it will be possible to teleport to a safe place should paralyze Dave's ability to teleport from the facility.